FIELD OF THE INVENTION
The invention relates to a transmission system comprising a data transmitter for supplying with a symbol interval data symbols to an input of a channel and a data receiver comprising a means for deriving a detection signal from an output signal of the channel, a means for deriving from the detection signal the most likely sequence of data symbols carried by the detection signal by recursively updating two candidate data sequences (survivors) on the basis of the value of a difference metric which is a measure for the probability difference of the survivors, the detection means furthermore including an adapting means for updating the difference metric, the new difference metric depending on a saturation function of the previous difference metric, and the saturation function for an ordinate value interval being linearly dependent on the ordinate value.
The invention likewise relates to a receiver to be used in a system of this type.
A transmission system as defined in the opening paragraph, as well as a receiver for this system is known from the journal article "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel" by R. G. Wood and D. A. Peterson in IEEE Transactions on Communications, Vol. COM-34, No. 5, May 1986.
Transmission systems of this type may be used, for example, for transmitting data signals through the public telephone network or for reconstructing data signals from a magnetic tape or disc.
When data symbols are transmitted via a transmission medium or stored on a recording medium respectively, the data symbols to be transmitted or recorded respectively, are converted into analog pulses which are consecutively fed to the transmission medium or recording medium respectively.
Generally, the analog pulses do not overlap in time. If the medium has a limited bandwidth, the pulses will start overlapping which in many cases will lead to the fact that the value of the received detection signal not only depends on a single data symbol at a given instant, but also on data symbols neighbouring in time. This effect is termed intersymbol interference.
Intersymbol interference may be caused not only by a limited bandwidth of the medium, but also by the use of a band limiting filter on the transmitter side which is used for giving a desired shape to the frequency spectrum of the transmitted or recorded analog pulses. The presence of intersymbol interference will often lead to an enhancement of the bit error rate.
An optimum receiver which could (substantially) completely eliminate the influence of intersymbol interference, could determine any possible sequence of transmitted data symbols and would determine the associated detection signal which would have been received if the relevant sequence of data symbols could have been transmitted through a noise-free channel. By comparing with the current detection signal all the detection signals thus obtained, the most likely sequence of transmitted data symbols could be determined. Such a receiver, however, would require an impracticably large computer and storage capacity.
In order to reduce this required computer and storage capacity, a commonly termed Viterbi detector is often used. In this detector the most likely sequence of transmitted data symbols is determined by recursively updating a limited number of M=L.sup.N-1 survivors, in which L is the number of levels of the transmission or recording signal used, and in which N is the channel impulse response length expressed in numbers of samples. This number is necessary, because the channel may occur in M states, whereas the receiver is to be capable of distinguishing between the states.
Once the M survivors with associated probability measures have been determined, each survivor, when a next data symbol is received, is extended and split up into a plurality of survivors of which only the most recently appended data symbols are different. The probability measure belonging to each new survivor is derived from the probability measure of the survivor from which the new survivor is derived and from an even function belonging to this new survivor of the difference between the current detection signal and the expected detection signal. Suitable even functions are the commonly termed L.sub.1 norm (.vertline.x.vertline.) and the L.sub.2 norm (x.sup.2) in which the L.sub.2 norm is used the most. For correctly determining the difference signal it is necessary for the amplitude of the detection signal to have a value that is used as a basis for determining this difference signal.
For achieving that the necessary storage capacity and computer capacity remains independent of the length of the sequence of transmitted data symbols, for each of the different possible channel states only the survivor is saved that is the most likely. Although a Viterbi detector already requires reduced computer and storage capacity, this capacity is nevertheless rather substantial.
In the aforementioned journal article a further simplification of the Viterbi detector is proposed which is possible if no more than two survivors need to be updated. This is the case if the number of possible values L of the transmission signal or recording signal is equal to two, and if the transfer is equal to 1+D or 1-D, where D represents a one symbol interval delay of the channel input signal. Since in this situation no more than two survivors need to be updated, it may be sufficient in lieu of updating a probability measure for each survivor, to update no more than one difference metric which is a measure for the difference between the two survivors. The known Viterbi detector, however, may also be used for channels having a transfer function of 1+D.sup.n or 1-D.sup.n. For this purpose, n such Viterbi detectors are to be used whilst samples of the detection signals are alternately applied to the various Viterbi detectors. This also results in the fact that the necessary operation rate of each Viterbi detector is reduced by a factor of n.
In the detection means known from above-mentioned journal article a new value of the difference metric belonging to a new value of the detection signal is equal to a saturation function of the difference between the old difference metric and a signal that is proportional to the detection signal. A saturation function in this context is meant to denote a function which in a certain ordinate value interval is (approximately) linear and which is substantially equal to a first or second saturation value respectively, for values of the ordinate situated on either one of the two sides of the interval. The saturation values are determined, for example, by the norm (for example, L.sub.1 or L.sub.2) used for determining the difference metric. Here too it is necessary for the amplitude of the detection signal to have a value that is used as a basis for determining this difference signal.
From the new value of the difference metric there can be directly determined how the survivors are to be extended. If the new value of the difference metric is situated in the (substantially) linear interval of the saturation function, the two survivors are to be extended by a symbol that has a value equal to the value of the most recent symbol of this survivor. In the two saturation regions in which the new difference metric may be situated, the new survivors are obtained by extending the first or second survivor to two new survivors by adding thereto the two possible symbol values. There has been assumed in this context that no precoding of the symbols to be transmitted or recorded respectively, has occurred, because when precoding is used there is a possibility of combining the inverse precoding with survivor generation.
A problem of the prior-art transmission system is that this transmission system presents only optimum performance with one specific value of the amplitude of the detection signal.